Try to look at it on a big screen, not sure about tablets but when I look at the pics from my 3+ black on my phone with go pro app they look pretty carappy but on my laptop directly from memory card they look good. So it maybe just the app if not then maybe theres something wrong with your camera and then if you contact Gopro they will exchange it for a new one, it happened to me with my first camera and within a week after getting RMA I got a new one. Good luck.
That too, it always look better on a nice sunny day but shouldn't be super bad on a cloudy one either. Most of my shots ware on cloudy days and it didn't come out too bad and you can alway do some post processing after too fix it a bit.
Try color correcting it, you can do it on the gopro cineform on the computer. Just do the auto color correction it should look better. I use after effects/premiere pro and when I color correct, it comes out really nicely.